4 ms·

Not really related, but even as a scientist I really don't understand how ng/L == parts-per-trillion. I know that's a correct conversion, but it doesn't doesn't

by sndean 10y ago

Not really related, but even as a scientist I really don't understand how ng/L == parts-per-trillion. I know that's a correct conversion, but it doesn't doesn't make much sense. I guess that's why part-per notation are called pseudo-units. A more substantive comment: Some of these are particularly scary, especially if you have a job where you're potentially exposed. For example, for benzene has a pretty strong odor, but 5000 ng/L is supposedly 1000 times below what you're able to smell. And it's a carcinogen...

sageikosa 10y ago
Some materials are reported in mass, some in volume. I used to do groundwater plume maps for superfund sites, and the data units varied by compound.
maxerickson 10y ago
It's because the liter was originally defined as 1 kilogram of water. For the various concentrations, the volume and mass of the displaced water is ~0.
infogulch 10y ago
This is because water has a nice correlation of units in SI. 1g water = 1 cubic cm = 1 mL, and takes 1 calorie of heat to raise by 1 degree Celsius. (all at atmospheric pressure). See: http://www.wolframalpha.com/input/?i=one+trillionth+of+a+liter+of+water+in+grams http://www.wolframalpha.com/input/?i=one+trillionth+of+a+lit...
daveguy 10y ago
The L part is assumed to have density of 1 (kg/L) and therefore be 1kg mass (mostly water). 1 kg = 1 trillion ng. Because the vast majority of the sample is water the number of nanograms of the substance per nanograms of water is approximately the number of parts per trillion. As you get into higher concentrations of solute you have to consider density of the solute and water as separate parts. In the ng/L or ug/L range you can hand wave the L as 1 trillion parts (by mass) and the ng as the parts out of a trillion in the full volume of sample.
MattSteelblade 10y ago
Not sure why parent comment put it in parts-per-trillion since I always see it in mg/L.

by sndean 10y ago

sageikosa 10y ago

Some materials are reported in mass, some in volume. I used to do groundwater plume maps for superfund sites, and the data units varied by compound.

maxerickson 10y ago

It's because the liter was originally defined as 1 kilogram of water. For the various concentrations, the volume and mass of the displaced water is ~0.

infogulch 10y ago

This is because water has a nice correlation of units in SI. 1g water = 1 cubic cm = 1 mL, and takes 1 calorie of heat to raise by 1 degree Celsius. (all at atmospheric pressure). See: http://www.wolframalpha.com/input/?i=one+trillionth+of+a+liter+of+water+in+grams http://www.wolframalpha.com/input/?i=one+trillionth+of+a+lit...

daveguy 10y ago

The L part is assumed to have density of 1 (kg/L) and therefore be 1kg mass (mostly water). 1 kg = 1 trillion ng. Because the vast majority of the sample is water the number of nanograms of the substance per nanograms of water is approximately the number of parts per trillion. As you get into higher concentrations of solute you have to consider density of the solute and water as separate parts. In the ng/L or ug/L range you can hand wave the L as 1 trillion parts (by mass) and the ng as the parts out of a trillion in the full volume of sample.

MattSteelblade 10y ago

Not sure why parent comment put it in parts-per-trillion since I always see it in mg/L.